Mechanical ventilation can be applied to a spontaneously breathing patient, for example through a mechanical ventilator. Then, the function of the mechanical ventilatory assist is to help the patient to maintain adequate ventilation by assisting the patient's weak respiratory muscles and by sharing the inspiratory load between the patient and the mechanical ventilator. When the mechanical ventilatory assist, expressed in terms of pressure, flow and/or volume, is delivered in synchrony with the patient's inspiratory effort, meaning that the mechanical ventilatory assist is delivered to the patient simultaneously with patient's inspiration, the mechanical ventilator will partially or completely overcome the inspiratory flow resistive and elastic loads induced by the respiratory system formed by the patient's lungs and thorax.
Furthermore, when the mechanical ventilator is neurally controlled as proposed in U.S. Pat. No. 5,820,560 granted to Sinderby et al on Oct. 13, 1998, the mechanical ventilatory assist can be delivered at the neural onset of the patient's inspiration so as to partially or completely overcome the inspiratory threshold load caused by the dynamic hyperinflation, also referred to as the intrinsic positive end-expiratory pressure (PEEP).
The currently used mechanical ventilators present a major limitation since they are unable to adequately determine the level of unloading of the patient's respiratory muscles or the patient's contribution to the inspiration when both mechanical ventilatory assist and the patient's own inspiratory muscles contribute to the inspiration. Even though methods for predicting unloading of the patient's respiratory muscles and the resistive and elastic load caused by the patient's respiratory system have been proposed, they still present the drawbacks of failing to show the patient's neural effort and neglecting the effect of the patient's muscle weakness and intrinsic PEEP.
Accordingly, there is a need for a system and method for determining a patient-ventilator breath contribution index in a spontaneously breathing, mechanically ventilated patient.